1. Field of the Invention
This invention relates generally to methods of producing structures and devices by use of techniques of nanotechnology. More specifically, but not exclusively, the invention relates to methods of producing nanostructures incorporating at least one element, essentially in one-dimensional form, and that is of nanometer dimensions in its width or diameter. For the purposes of this application, such an element will be termed a “nanowhisker”. The invention also relates to structures and devices incorporating nanowhiskers, preferably produced by the so-called Vapor-Liquid-Solid (VLS) mechanism.
2. Background Art
Nanotechnology techniques and processes may provide structures that range in size from small devices of atomic dimensions, to much larger scale structures for example, on the microscopic scale. Commonly, such structures include “nanostructures”. In certain contexts nanostructures are considered to be those having at least two dimensions not greater than about 100 nm. Ordinarily, layered structures or stock materials having one or more layers with a thickness less than 1 μm are not considered to be nanostructures. Nanostructures include one-dimensional nanoelements, essentially in one-dimensional form, that are of nanometer dimensions in their width or diameter, and that are commonly known as nanowhiskers, nanorods, nanowires, nanotubes, etc.
The basic process of microscopic whisker formation on substrates, by the so-called VLS (vapor-liquid-solid) mechanism, is well known. A particle of a catalytic material, usually gold, for example, on a substrate is heated in the presence of certain gases to form a melt. A pillar forms under the melt, and the melt rises up on top of the pillar. The result is a whisker of a desired material with the solidified particle melt positioned on top. See E. I Givargizov, Current Topics in Materials Science, Vol. 1, pages 79-145, North Holland Publishing Company, 1978.) The dimensions of such whiskers were in the micrometer range.
International Application Publication No. WO 01/84238 discloses in FIGS. 15 and 16 a method of forming nanowhiskers, wherein nanometer sized particles from an aerosol are deposited on a substrate and these particles are used as seeds to create nanowhiskers, and other one-dimensional nanoelements.
Although the growth of nanowhiskers catalysed by the presence of a catalytic particle at the tip of the growing whisker has conventionally been referred to as the VLS (Vapor-Liquid-Solid) process, it has come to be recognized that the catalytic particle may not have to be in the liquid state to function as an effective catalyst for whisker growth. At least some evidence suggests that material for forming the whisker can reach the particle-whisker interface and contribute to the growing whisker even if the catalytic particle is at a temperature below its melting point and presumably in the solid state. Under such conditions, the growth material, e.g., atoms that are added to the tip of the whisker as it grows, may be able to diffuse through a the body of a solid catalytic particle or may even diffuse along the surface of the solid catalytic particle to the growing tip of the whisker at the growing temperature. Evidently, the overall effect is the same, i.e., elongation of the whisker catalysed by the catalytic particle, whatever the exact mechanism may be under particular circumstances of temperature, catalytic particle composition, intended composition of the whisker, or other conditions relevant to whisker growth. For purposes of this application, the term “VLS process”, or “VLS mechanism”, or equivalent terminology, is intended to include all such catalysed procedures wherein nanowhisker growth is catalysed by a particle, liquid or solid, in contact with the growing tip of the nanowhisker.
As previously noted, for the purposes of this application the term nanowhiskers is intended to mean one-dimensional nanoelements with a width or diameter (or, generally, a cross-dimension) of nanometer size. Preferably, although not necessarily, the elements are formed by the so-called VLS mechanism. Nanowhiskers are also referred to in the art as “nanowires” or, in context, simply as “wires”, and such terminology, as used in this application, is equivalent to the term “nanowhiskers”. In preferred practice of the invention, nanowhisker cross-dimensions typically do not exceed 50 nm and more preferably range about 20 nm or less. However, it is within the broader scope of the invention to utilize nanowhiskers having cross-dimensions greater than 50 nm, say up to 100 nm, or even 500 nm or more. Of course, larger nanowhisker cross-dimensions will generally result in larger structures and may therefore be less desirable in many applications.
Several experimental studies on the growth of nanowhiskers have been made. Hiruma et al. grew III-V nanowhiskers on III-V substrates in a metal organic chemical vapor deposition (MOCVD) growth system. See K. Hiruma, et al., J. Appl. Phys. 74, page 3162 (1993); K. Hiruma, et al., J. Appl. Phys. 77, page 447 (1995); K. Hiruma, et al., IEICE Trans. Electron. E77C, page 1420 (1994); K. Hiruma, et al., J. Crystal Growth 163, pages 226-231 (1996).
In prior U.S. patent application Ser. No. 10/613,071, published as No. 2004-0075464, to Samuelson et al., and International Patent Application Publication No. WO-A-04/004927, both of which publications and their underlying applications are incorporated herein by reference, there are disclosed methods of forming nanowhiskers by a chemical beam epitaxy method. Nanowhiskers are disclosed having segments of different materials, with abrupt or sharp heterojunctions therebetween. Various nanostructures of controlled size and formation are disclosed. Such nanostructures serve as components in the formation of novel structures, as disclosed.
Gao et al., “Self-assembled Nanowire—Nanoribbon Junction Arrays of ZnO”, J. Phys. Chem. B, vol. 106(49), pages 12653-12658 discloses nanostructures comprising branched structures of ZnO, formed by a VLS process.
Nanocrystals have been synthesised with a wide variety of shapes. Synthesis of tetrapod-shaped structures of controlled size as integral structures by precipitation from solutions is disclosed in Manna et al., “Synthesis of soluble and processable rod-, arrow-, teardrop-, and tetrapod-shaped CdSe nanocrystals”, J. Am. Chem. Soc. 122, pages 12700-12706 (2000).
As regards structures formed of large numbers of nanostructures, solar cells (so-called Grätzel cells) are known comprising optically transparent films of TiO2 particles coated with monolayers of a charge transfer dye. See O'Regan et al., “A low-cost, high-efficiency solar cell based on dye-sensitive colloidal TiO2 films”, Nature 353, pages 737-740 (1991).
Much research has been carried out into neural networks (neuromorphic electronics) since the pioneering work of Carver Mead with VLSI systems. For a recent report, see, for example, Cohen et al., Report on the 2003 Workshop on Neuromorphic Engineering, Telluride, Colo., Jun. 29-Jul. 19, 2003. However, techniques of nanotechnology have not generally been applied in this area.